Investments in Unconsolidated Entities - White Mountain's Investments (Details) - USD ($) $ in Millions		3 Months Ended
	May 07, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	May 31, 2019
Schedule of Equity Method Investments [Line Items]
Stock Issued During Period, Value, Acquisitions	$ 15.0
Ownership interest (as a percent)		53.80%	50.00%	50.00%
Passport/DavidShield [Member]
Schedule of Equity Method Investments [Line Items]
Stock Issued During Period, Value, Acquisitions		$ 15.0
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		48.30%		48.30%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		45.00%		45.00%
Elementum Holdings, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		30.00%		30.00%	30.00%
JAM Partners L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)				11.10%
Minimum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		14.30%		17.50%
Minimum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		9.40%		10.00%
Minimum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		3.20%		3.20%
Maximum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		62.30%		62.40%
Maximum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		36.50%		38.40%
Maximum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		35.00%		30.00%